This invention relates to flame retardant molding compositions.
Epoxy resin is a widely used molding compounds for coating electronic devices such as integrated circuits. For safety reasons, molding compositions containing epoxy resin often include flame retardants. A common flame retardant system is a combination of bromine containing flame retardants and antimony oxide flame retardant synergists. However, these compounds are pollutants of the environment. Some bromine containing flame retardants (especially brominated diphenyl ethers) are toxic and possibly carcinogenic. As to antimony trioxide, it is classified by the International Agency for Research on Cancer as a Class 2B carcinogen (i.e., antimony trioxide is a suspect carcinogen based mainly on animal studies). In addition, this compound is often used at relatively high level (2-4%) and is also slightly water-soluble, leading to further environmental concerns. This concern is highlighted by the fact that integrated circuit manufacturers currently discard up to one half of the total amount of molding compositions used as waste product in landfills.
Phosphorus-containing compounds have been proposed as flame retardants. Although they are less hazardous, molding compositions containing these compounds generally possess undesirable properties such as high moisture absorption rate. Thus, there exists a need to develop new flame retardant molding compositions that do not contain brominated flame retardants, phosphorus-containing compounds, or antimony oxide flame retardant synergists.
In general, the invention relates to flame retardant molding compositions that are substantially free of halogen, phosphorus, and antimony. In addition to having good flame retarding properties, these compositions form good cull cure in short time periods and absorb low amount of moisture, and can be used to coat electronic or electrical devices such as semiconductors, diodes, and integrated circuits. Such coated devices demonstrate good electrical reliability at high temperature.
In one aspect, the invention features a flame retardant molding composition that is substantially free of halogen, phosphorus, and antimony. The molding composition includes an epoxy resin, a first transition metal oxide containing a refractory metal, and a second transition metal oxide containing an oxyanion of a Group VIA element.
In another aspect, the invention features a flame retardant molding composition substantially free of halogen, phosphorus, and antimony, including an epoxy resin, a phenol novalac hardener containing a biphenyl or naphthyl moiety, and a transition metal oxide of a Group VIA element.
In another aspect, the invention features a flame retardant molding composition that is substantially free of halogen, phosphorus, and antimony, including an epoxy resin containing a biphenyl or naphthyl moiety, a phenol novalac hardener, and a transition metal oxide of a Group VIA element.
The invention also features a method of preparing a flame retardant polymer composition that is free of halogen, phosphorus, and antimony. The method includes heating a molding composition to a temperature sufficient to cure the molding composition (e.g., about 150xc2x0 C. to about 200xc2x0 C., or about 165xc2x0 C. to about 195xc2x0 C.). The molding composition cures in about 1 minute to about 2 minutes, and contains an epoxy resin, a first transition metal oxide containing a refractory metal, and a second transition metal oxide containing an oxyanion of a Group VIA element. The invention also features polymer compositions formed by this method.
The invention also features a method of coating an electrical or electronic device such as an integrated circuit. The method includes heating a molding composition to a temperature sufficient to cure the molding composition (e.g., about 150xc2x0 C. to about 200xc2x0 C., or about 165xc2x0 C. to about 195xc2x0 C.). The polymer composition thus formed coats the surface of the device. The molding composition contains an epoxy resin, a first transition metal oxide containing a refractory metal, and a second transition metal oxide containing an oxyanion of a Group VIA element. The invention also features coated devices prepared by this method.
As used herein, a composition that is xe2x80x9csubstantially freexe2x80x9d of a material means that the amount of the material is negligible in the composition, i.e., less than about 0.001 wt % of the total weight of the composition.
As used herein, a refractory metal is a metal having a melting point of around 2,000xc2x0 C. or above. Some examples of a refractory metal are zirconium, niobium, molybdenum, ruthenium, iridium, hafnium, tantalum, tungsten, osmium, vanadium, chromium, rhenium, and rhodium.
As used herein, an oxyanion is a polyatomic anion containing oxygen, e.g., molybdate and chromate.
As used herein, a compound is water-insoluble when it has a solubility of less than 0.05 g in 100 mL of water at 25xc2x0 C.
As used herein, a molding composition is cured when it forms a good cull cure (i.e., strong and not brittle).
Other features and advantages of the invention will be apparent from the description of the preferred embodiments thereof, and from the claims.
A preferred molding composition contains an epoxy resin, a hardener, and two transition metal oxides, and optionally a third transition metal oxide.
There is no restriction on the type of epoxy resin that can be used in the molding compositions so long as it contains two or more reactive oxirane groups. Some suitable epoxy resins are epoxy cresol novalac resin, biphenyl epoxy resin, hydroquinone epoxy resin, phenolic novalac epoxy resin, and stilbene epoxy resin. Epoxy cresol novalac resin is preferred. The molding compositions can include more than one epoxy resin, for example, a combination of epoxy cresol novalac resin and biphenyl epoxy resin. The preferred weight percent of the epoxy resin ranges from 4 wt % to about 12 wt %, and more preferably, from about 5.5 wt % to about 8.5 wt %, based on the total weight of the molding composition.
The hardener promotes crosslinking of the molding composition to form a polymer composition. Some suitable hardeners that can be included in the molding compositions are phenol novalac hardener, cresol novalac hardener, dicyclopentadiene phenol hardener, and limonene type hardener. Phenol novalac hardener is preferred. Similar to the epoxy resin component, more than one type of hardener can be included in the molding compositions. The preferred weight percent of the hardener ranges from 1 wt % to about 10 wt %, and more preferably, from about 1.5 wt % to about 6 wt %, based on the total weight of the molding composition.
As to the two transition metal oxides, the first transition metal oxide contains a refractory metal, e.g., chromium, molybdenum, and tungsten, and the second transition metal oxide contains an oxyanion of a Group VIA element, e.g., molydate and tungstate. While there is no particular restriction on the cation of the second transition metal oxide, it is preferred to be a Group IIB metal cation, e.g., zinc. Both the first and second transition metal oxides are preferred to be water-insoluble. A particularly preferred first and second transition metal oxides are tungsten trioxide and zinc molybdate, respectively. The transition metal oxides are preferred to be in their free form, i.e., they are not associated with materials such as silica or talc. The oxides are also preferred to be finely divided, e.g., having a diameter of about 0.1 xcexcm to about 10 xcexcm, preferably, about 0.5 xcexcm to about 5 xcexcm, or more preferably, about 0.5 xcexcm to about 2 xcexcm. The oxides can be obtained commercially, e.g., tungsten trioxide and zinc molybdate are available from Aldrich Chemical Company (Milwaukee, Wis.) and the Sherwin-Williams Company (Cleveland, Ohio), respectively. The molding composition can include, for example, about 0.25 wt % to about 2 wt %, preferably about 0.5 wt % to about 1 wt %, and more preferably about 0.75 wt % of the first transition metal oxide based on the total weight of the molding composition. As to the second transition metal oxide, the molding composition can include, for example, about 0.75 wt % to about 6 wt %, preferably about 1 wt % to about 4 wt %, and more preferably about 3 wt % based on the total weight of the molding composition.
The preferred molding compositions can include a third transition metal oxide of Group VIA element. An example of such a metal oxide is molybdenum trioxide. The weight percent of the third transition metal oxide in the molding compositions can range from about 0.1 wt % to about 1 wt %, and preferably about 0.5 wt % to about 1 wt %, and more preferably about 0.75 wt % based on the total weight of the molding composition.
Another preferred molding composition contains an epoxy resin, a phenol novalac hardener containing a biphenyl or naphthyl moiety, and a transition metal oxide of a Group VIA element. Preferably, the phenol novalac hardener is the only component of the molding composition that contains a biphenyl or naphthyl moiety. More preferably, the composition is substantially free of epoxy resins other than the cresol novalac type.
The preferred weight percent of the epoxy resin ranges from 4 wt % to about 12 wt %, and more preferably, from about 5.5 wt % to about 8.5 wt %, based on the total weight of the molding composition.
There is no particular restriction on the phenol novalac hardener so long as it contains a biphenyl or naphthyl moiety. The phenolic hydroxy groups can be attached to the biphenyl or naphthyl moiety of the hardener. A preferred phenol novalac hardener is commercially available from Meiwa Plastic Industries, Ltd., Japan (catalog no. MEH 7851, SS graded). This type of hardener can also be prepared according to the methods described in EP 915 118 A1. For example, a hardener containing a biphenyl moiety can be prepared by reacting phenol with bismethoxy-methylenebiphenyl. The weight percent of the phenol novalac hardener containing a biphenyl or naphthyl moiety can range from 1 wt % to about 10 wt %, and more preferably, from about 1 wt % to about 8 wt %, based on the total weight of the molding composition.
Examples of a transition metal oxide of a Group VIA element include oxides of chromium, molybdenum, and tungsten, with tungsten trioxide being the preferred oxide. The molding composition can include, for example, about 0.25 wt % to about 2 wt %, preferably about 0.5 wt % to about 1 wt %, and more preferably about 0.75 wt % of the transition metal oxide based on the total weight of the molding composition.
Yet another preferred molding composition contains an epoxy resin containing a biphenyl or naphthyl moiety, a phenol novalac hardener, and a transition metal oxide of a Group VIA element. Preferably, the epoxy resin is the only component of the molding composition that contains a biphenyl or naphthyl moiety.
The epoxy resin containing a biphenyl or naphthyl moiety can be obtained commercially, e.g., from Nippon Kayaku Co., Ltd., Japan (catalog no. NC-3000P). The preparation of this type of epoxy resin is also described in EP 915 118 A1. For example, an epoxy resin containing a biphenyl moiety can be prepared by reacting phenol with bismethoxy-methylenebiphenyl, followed by treatment with a glycidyl compound such as glycidyl tosylate to form the desired epoxy resin. The preferred weight percent of the epoxy resin containing a biphenyl or naphthyl moiety ranges from 4 wt % to about 12 wt %, and more preferably, from about 5.5 wt % to about 8.5 wt %, based on the total weight of the molding composition.
The preferred weight percent of the phenol novalac hardener ranges from 1 wt % to about 10 wt %, and more preferably, from about 1 wt % to about 8 wt %, based on the total weight of the molding composition.
As described above, a transition metal oxide of a Group VIA element can be an oxide of chromium, molybdenum, and tungsten. Tungsten trioxide is preferred. The molding composition can include, for example, about 0.25 wt % to about 2 wt %, preferably about 0.5 wt % to about 1 wt %, and more preferably about 0.75 wt % of the transition metal oxide based on the total weight of the molding composition.
The molding compositions of this invention can include other additives (the wt % is calculated based on the total weight of the molding composition):
a filler such as silica, calcium silicate, and aluminum oxide (the preferred molding composition can contain 50-95 wt %, more preferably, 60-90 wt % of a filler);
a colorant such as carbon black colorant (the preferred molding composition can contain 0.1-2 wt %, more preferably, 0.1-1 wt % of a filler);
a mold release agent such as carnauba wax, paraffin wax, polyethylene wax, glycerol monostearate, and metallic stearates (the preferred molding composition can contain 0.1-2 wt %, more preferably, 0.2-1 wt % of a mold release agent);
fumed silica such as aerosil (the preferred molding composition can contain 0.3-5 wt %, more preferably, 0.7-3 wt % of a fumed silica);
a coupling agent such as the silane type coupling agent (the preferred molding composition can contain 0.1-2 wt %, more preferably, 0.3-1 wt % of a coupling agent);
a catalyst such as 1,8-diazabicyclo-(5,4,0)undecene-7-triphenylphosphone and 2-methylimidazole (the preferred molding composition can contain 0.1-10 wt %, more preferably, 0.5-2 wt % of a catalyst); and
ion scavengers such as magnesium aluminum carbonate hydrate, which can be obtained commercially from Kyowa Chemical Industry Co. under the trade name xe2x80x9cDHT-4Axe2x80x9d (the preferred molding composition can contain 0.1-2 wt %, more preferably, 0.5-2 wt % of an ion scavenger).
The molding compositions can be prepared by any conventional method. For example, U.S. Pat. No. 5,476,716 teaches a method of finely grounding, dry blending, and then densifying all of the components of a molding composition on a hot differential roll mill, followed by granulation. Also described in the patent are methods of preparing a molding composition on a laboratory or a pilot plant scale. Alternatively, one can mix the components of a molding composition in a stepwise fashion to enhance homogeneous mixing. Specifically, the first step of the method involves mixing and heating the epoxy resin and the hardener until melting occurs (around 150xc2x0 C.). The transition metal oxide is then added to the resin and hardener to form a mixture, which is then blended with a mixer until thoroughly mixed (for around 10 minutes). The mixture is allowed to cool until hardened before grounding to a fine powder. The powder is then added to the rest of the components of the molding composition and dry blended before milling. For example, a large two-roll mill (one roll heated to about 90xc2x0 C., and the other cooled with tap water) can be used to produce uniform sheets, which are then grounded to powder after cooling.
The molding compositions can be molded into various articles by any conventional method, e.g., by using molding apparatus such as a transfer press equipped with a multi-cavity mold for coating electronic devices. Suitable molding conditions include a temperature of about 150xc2x0 C. to about 200xc2x0 C. (preferably about 175xc2x0 C. to about 195xc2x0 C.) a pressure of about 400 psi to about 1,500 psi.
The preferred molding compositions cure in about 0.5 minute to about 3 minutes, more preferably, about 1 minute to about 2 minutes. To determine the time for curing (i.e., minimum time needed for forming a good cull cure), the molding composition is placed in the mold press at 190xc2x0 C. and is inspected after a pre-set period of time (e.g., 3 minutes). If a good cure (i.e., strong and not brittle) is formed, the experiment is repeated with a shorter period of press time until the minimum time period is determined.
The preferred molding compositions demonstrate a flammability rating of UL 94V-1, more preferably, a flammability rating of UL 94V-0. The ratings are determined by measuring the total burn time of a xe2x85x9xe2x80x3 bar according to the UL 94 flammability test. A 94V-0 and a 94V-1 rating require the total burn time for a single bar to be less than or equal to 10s and 30s, respectively.
Preferably, the inclusion of transition metal oxides in the molding compositions does not increase the rate of moisture absorption, which is determined by a method similar to ASTM D570-95. Briefly, the procedure involves placing a weighed a molded disk of 3xe2x80x3 diameter and xe2x85x9xe2x80x3 thick in a rack in an upright position. The rack is then placed on a platform in an 85xc2x0 C., 85% relative humidity chamber for a pre-determined time period, and the disk is weighed afterwards. The % weight gain is determined by multiplying the quotient (weight difference of the bar before and after placing in the chamber/initial weight of the bar) by 100%.
The electrical reliability of the coated devices in a moist environment is determined by placing the coated devices, e.g., integrated circuits, with no bias in an autoclave at 121xc2x0 C., 15 psi, and 100% relative humidity. After a number of hours, the coated devices are dried and tested with an electrical tester. The number of coated devices showing a failure in any one of several electrical parameters is counted. These parameters, set by the manufacturer of the device, include, for example, the net DC input offset current for zero device output, the current from device negative input with zero output, the current from the device positive input terminal with zero output, the average of the two previous parameters, DC input offset voltage for zero device output, etc. The % failure is calculated by multiplying the quotient (number of failed devices/total number of tested devices) by 100%. The preferred molding compositions have good electrical reliability in a moist environment, i.e., less than 50% failure after 1,000 hours under conditions as described above.
The high temperature storage life (HTSL) test assesses electrical reliability of the coated devices in a dry environment (i.e., room humidity) and atmospheric pressure. In the HTSL test, parametric shifts in voltage output levels are monitored, and the coated devices are stored at 200xc2x0 C. The voltage output levels reflect increased resistance across the ball-bonds of the devices. The preferred molding compositions delay or eliminate the failure due to parameter shifts in voltage output levels of coated devices. Similar to the autoclave test, the % failure is calculated by multiplying the quotient (number of failed devices/total number of tested devices) by 100%. The preferred molding compositions have good electrical reliability in a dry environment, i.e., less than 50% failure after 1,500 hours under conditions as described above.